1. Field of the Invention
The invention relates to a rear-projection system, comprising an enclosure which accomodates at least one image source, a corresponding number of projection-lens systems, and a rear-projection screen.
2. Description of the Prior Art
In a rear-projection system, a projection lens system projects the primary image produced by an image source onto a first side, referred to as the rear, of the projection screen. This image can be observed by a viewer in front of the system, looking toward the second or front side of the screen.
Rear-projection systems comprising one image source in the form of a cathode-ray tube for monochrome reproduction, or three such image sources for color reproduction, are employed for displaying a video program with a picture size which is substantially larger than the size attainable with a conventional television picture tube. The projection system may also be employed for displaying other information, such as visual information associated with speech, messages to visitors of exhibitions, etc. An image source used in the projection system may also comprise a light source and a transparency in the form of a slide or film arranged in front of the light source. Alternatively, an image source may comprise a light source and an electronically controllable flat display device comprising a matrix of picture cells which, depending on the electronic signal, reflect or absorb light, or transmit or absorb light. Such a flat display device for example enables video pictures to be displayed.
The rear-projection arrangements described above are intended for use in an illuminated environment such as a room, an exhibition space or an auditorium. In order to ensure that the projected image has a satisfactory brightness and contrast, a maximum amount of the light produced by the image source, referred as signal light, should be transmitted by the screen, and conversely; a minimal amount of ambient light should be reflected towards the viewers by the screen and other elements of the projection system.
In the rear-projection systems known until now it has been attempted to achieve this by a concentrated emission of signal light from surface portions of the screen and by making the other surface portions light-absorbing. As a result, one portion of ambient lighting impinging on the screen is absorbed, another portion is transmitted into the enclosure, and the remainder is reflected by the screen. An example of such a projection screen is the system described in U.S. Pat. No. 3,832,032. The rear surface of the projection screen used in this system is provided with a large number of cylindrical lenses which extend in a vertical direction, and which concentrate the light originating from the image source(s) in vertical bands on the front of the screen. The stripes between these bands are provided with a light-absorbing layer. This screen provides a substantial increase in the ratio between the signal light emerging from the screen and the ambient light reflected by the screen, as can be observed by the viewer.
However, ambient light which enters the screen via the light-transmitting bands between the light-absorbing stripes can emerge again, for example, as a result of total internal reflection from the inside of the cylindrical lenses. When the light-absorbing material covers 50% of the surface area this effect still results in 10% of the ambient light being reflected. As the light-absorbing material is situated at the front of the screen, this material is exposed to damaging external mechanical and chemical influences.